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In this paper, Cu–Cu interconnects with ultrafine pad pitches of 6 μm, 8 μm, and 15 μm are implemented on the12 inch wafers by a direct bonding process. Defects are not found by traditional non-destructive(NDT) c-mode scanning acoustic microscopy(c-SAM). However, cross sectional observation of bonding interfaces reveals that micro-defects such as micro seams are located at SiO_2 bonding interfaces. In order to examine the micro-defects in the ultra-fine pitch direct bonding process by the NDT technology, a novel “defect-enlarged approach” is proposed. The bonded dies are first annealed in an N_2 oven at 300?C for a few hours and then cooled quickly in air. The c-SAM scanning images show large defects at the place where nothing can be detected by c-SAM before this treatment. Cross sectional observation of the bonding interfaces indicates that these defects consist of large size micro seams at the SiO_2 bonding interface, especially near Cu pads with an ultrafine pitch of 6 μm. However, these large defects disappear after several hours at room temperature, observed by c-SAM. It is inferred that the disappearance of these defects inspected by the “defect-enlarged approach” results from the combination of intrinsic micro seams and “weak” bonds in the silicon oxide layer. Then the underlying physical mechanism of these micro-defects is proposed, which is influenced by Cu pad surface topology and bonding models.
In this paper, Cu-Cu interconnects with ultrafine pad pitches of 6 μm, 8 μm, and 15 μm are implemented on the 12 inch wafers by a direct bonding process. Defects are not found by traditional non-destructive (NDT) c-mode scanning However, cross sectional observation of bonding interfaces reveals that micro-defects such as micro seams are located at SiO 2 bonding interfaces. In order to examine the micro-defects in the ultra-fine pitch direct bonding process by the NDT technology, a novel “defect-enlarged approach” is proposed. The bonded dies are first annealed in an N_2 oven at 300 ° C for a few hours and then cooled quickly in air. The c-SAM scanning images show large defects at the place where nothing can be detected by c-SAM before this treatment. Cross sectional observation of the bonding interfaces indicates that these defects consist of large size micro seams at the SiO 2 bonding interface, especially near Cu pads with an ultrafine pitch of 6 μm. H owever, these large defects disappear after several hours at room temperature, observed by c-SAM. It is inferred that the disappearance of these defects is inspected by the “defect-enlarged approach ” results from the combination of intrinsic micro seams and Then the underlying physical mechanism of these micro-defects is proposed, which is influenced by Cu pad surface topology and bonding models.